Schizophrenia and Human Self-Domestication: An Evolutionary Linguistics Approach

Abstract

Schizophrenia (SZ) is a pervasive neurodevelopmental disorder that entails social and cognitive deficits, including marked language problems. Its complex multifactorial etiopathogenesis, including genetic and environmental factors, is still widely uncertain. SZ incidence has always been high and quite stable in human populations, across time and regardless of cultural implications, for unclear reasons. It has been hypothesized that SZ pathophysiology may involve the biological components that changed during the recent human evolutionary history, and led to our distinctive mode of cognition, which includes language skills. In this paper we explore this hypothesis, focusing on the self-domestication of the human species. This has been claimed to account for many human-specific distinctive traits, including aspects of our behavior and cognition, and to favor the emergence of complex languages through cultural evolution. The “domestication syndrome” in mammals comprises the constellation of traits exhibited by domesticated strains, seemingly resulting from the hypofunction of the neural crest. It is our intention to show that people with SZ exhibit more marked domesticated traits at the morphological, physiological, and behavioral levels. We also show that genes involved in domestication and neural crest development and function comprise nearly 20% of SZ candidates, most of which exhibit altered expression profiles in the brain of SZ patients, specifically in areas involved in language processing. Based on these observations, we conclude that SZ may represent an abnormal ontogenetic itinerary for the human faculty of language, resulting, at least in part, from changes in genes important for the domestication syndrome and primarily involving the neural crest.

Abnormal features of human self-domestication in bipolar disorder

“…Overall, our findings suggest that the complex phenotype exhibited by people with BD can be construed as a combination of hypo- and hyperdomesticated features. This is in line with the mixed nature of this disorder, entailing both schizophrenia-like features (with SZ being in turn a hyperdomesticated human phenotype; see Benítez-Burraco et al, 2017 for discussion) and ASD-like features (with ASD being in turn a hypodomesticated human phenotype; see Benítez-Burraco et al, 2016 for discussion). This is also in line with previous accounts of BD as an intermediate phenotype within the continuum of human social cognition (Crespi and Badcock, 2008).…”

“…In further support of this view, regions positively selected in modern humans compared to extinct hominins are enriched in candidate genes for domestication in mammals (Theofanopoulou et al, 2017). Interestingly, complex genetic conditions involving cognitive deficits, behavioral anomalies, and abnormal socialization patterns, like SZ and ASD, are characterized by abnormal presentations of traits associated with (self-)domestication; moreover, candidate genes for domestication are overrepresented among the candidates for these conditions and/or exhibit altered expression profiles in the brains of affected people (Benítez-Burraco et al, 2016; Benítez-Burraco et al, 2017; Benítez-Burraco, 2020b). But the same pattern holds in conditions with a neat molecular etiology, like WS, as genes outside the affected genomic region that are differentially-expressed in patients are likewise enriched in candidates for domestication (Niego and Benítez-Burraco, 2019).…”

Recent selection of candidate genes for mammal domestication in Europeans and language change in Europe: a hypothesis

“…Our four different sets of candidates for domestication resulted from i) merging the list we compiled for our paper on features of domestication in schizophrenia (Benítez-Burraco et al 2017) with the list compiled by Theofanopoulou and collaborators (2017). The merged list includes 764 genes ( Supplemental table 1).…”

“…We also considered ii) the 41 genes highlighted by Theofanopoulou and collaborators as showing evidence of positive selection in modern humans compared to Neanderthals/Denisovans ( Supplemental table 1). In view of the suggested role of the NC in the emergence of features of domestication, we considered as well iii) genes important for NC development and function, which we also compiled for our paper on domestication and schizophrenia (Benítez-Burraco et al, 2017). This list encompasses 89 genes ( Supplemental table 1), which we gathered using pathogenic and functional criteria: neurochristopathy-associated genes annotated in the OMIM database (http://omim.org/), NC markers, genes that are functionally involved in NC induction and specification, genes involved in NC signaling (within NC-derived structures), and genes involved in cranial NC differentiation.…”

“…Signs of self-domestication in humans have increased in recent times (reviewed in Hare, 2017). Interestingly too, features of domestication are found abnormal (either exacerbated or attenuated) in clinical conditions impacting on our cognitive abilities, including language, like autism spectrum disorder (Benítez-Burraco et al, 2016) schizophrenia (Benítez-Burraco et al, 2017), or Williams syndrome (Niego and Benítez-Burraco, 2019). At the same time, genomic regions associated with dog-human communication contain genes related to human social disorders, particularly autism spectrum disorder (Persson et al, 2016).…”